Cognates are words in different languages that are similar in spelling. They can be TRUE or FALSE.
True cognates have the same meaning in both languages, whereas false cognates have different meaning.
For example: the words NAÇÃO and NATION are similar, so they are COGNATES. They have the same meaning, "A large body of people united by common descent, history, culture, or language, inhabiting a particular state or territory (1)". So they are true.
Some books call cognates "friends", so they can be "true friends" or "false friends".
As English and Portuguese share about 58% of their vocabulary, i.e. 58% of English and Portuguese words are similar, and only 1,8% of these cognates are false, using cognates is a tool or strategy that you can use to improve your vocabulary, or, in case you forget, you can try to use a cognate - you have 98% of chances to be right!!!
(1) Oxford Dictionaries. Available at <http://www.oxforddictionaries.com/definition/english/nation>
TASK
Here's a task for you:
Watch this PowerPoint presentation and make a list of cognates from all the categories shown in the presentation
_dade _ty
cidade city
velocidade velocity
etc
_ção _tion
simplificação simplification
nação nation
etc